Dual band antennas have many applications. For example, systems in which transmit and receive modes are separated in bandwidth are in use or being proposed.
In systems that feature dual band operation, it is desirable to provide a single antenna aperture that supports both the transmit and receive modes. In order to operate an antenna at multiple frequency bands, diplexers have been used. In concept, diplexers separate the bandwidth of a wide band radiating structure into two narrower bands. Diplexers typically comprise filters that selectively feed low and high frequency radiating elements, and can be difficult and expensive to implement. In addition, diplexers can introduce losses, take up a significant amount of space, and add complexity and mass to an antenna assembly. Moreover, it is difficult to obtain sufficient isolation between operational bandwidths using traditional diplexer architectures.
Although diplexers have a number of shortcomings, their use is typically required in order to support dual band operation. In particular, coupling between the feeds of a dual band system limits the amount of isolation between the frequency bands. Accordingly, the user of diplexers, which take up significant space, as well as adding cost and complexity, has often been unavoidable.
In order to provide isolation between differently polarized radiators, designs have been developed that do not require separate filters in order to achieve such isolation. For example, high isolation between the input/output port for a first polarization with respect to the input/output port for an orthogonal polarization can be achieved by simultaneously feeding a pair of patches such that a portion of a first signal provided at a first input/output port destructively interferes with a second portion of that signal at the second input/output port. Such a system is described in U.S. Pat. No. 4,464,663, the entire disclosure of which is hereby incorporated herein by reference. However, that solution, which involves feeding a plurality of patches from first and second feed line systems is not applicable to systems in which different feed lines are used to supply signals at different bandwidths to different radiating elements.
Accordingly, it would be desirable to provide a dual band antenna system that provided acceptable isolation between the bands, and that avoided the need for complex filters.